The translocation of calcium from sarcoplasmic reticulum to myofibrils is the critical link in excitation-contraction coupling in striated muscle. The activating effect of calcium on the contractile mechanism is mediated through a regulatory protein complex bound to the actin filament. This complex consists of tropomyosin and troponin. The latter is composed of an inhibitory component, troponin B, and a calcium binding component, troponin A. The binding of calcium to troponin A suppresses the inhibitory action of the tropomyosin-troponin system and allows for the ATP-dependent interaction between the contractile proteins, actin and myosin. The aims of the proposed research are to elucidate the properties of the calcium receptor site and to determine the molecular events which intervene between the binding of calcium and the onset of tension development. Studies will be carried out of the in vitro interaction between calcium and purified components of the regulatory protein complex. Physicochemical features of calcium-troponin interaction will be analyzed through studies of cation competition, the biological properties of substituted metal-troponin complexes, and the effects of chemical and enzymatic modification on calcium binding. The components of the regulatory complex will be selectively modified and recombined to determine the localization of functional groups involved in interactions of the regulatory proteins with each other and with the contractile proteins. The effect of calcium on protein-protein interactions will be studied by electrophoretic, viscometric, turbidimetric, and chemical methods.